In the recovery of oil from a subterranean oil-bearing formation only a limited amount of the oil in place is recoverable by use of primary and secondary recovery processes. Hence, several tertiary or enhanced oil recovery processes have been developed. Such processes include thermal processes, exemplary of which are steam flooding and in-situ combustion, chemical flooding techniques and gaseous displacement fluid recovery methods. The gases utilized include steam, carbon dioxide, nitrogen or hydrocarbons. The present invention has to do with improving gaseous displacement fluid recovery methods.
There are problems associated with the use of gaseous displacement fluid recovery methods. First, fingering of the gas phase into the oil, with attendant degradation of the uniform displacement front, may take place. This is because of the adverse mobility ratio between the displacing gas and the oil. The second problem is gravity override which is promoted by the density difference between the gas and oil phases. Thus because the sweep efficiency, or contact between the injected fluid and the oil in the reservoir, is reduced because of these problems, the incremental recovery will as a result also be reduced. Reservoir heterogeneity will further compound these problems by promoting channelling, thereby further reducing the sweep efficiency.
Foams have been emplaced in a reservoir as a means for combatting fingering or gravity override by the gaseous displacement fluid. Such foams are normally formed using a gas, a surfactant and a liquid. The foam is either generated in-situ by injecting the ingredients into the formation or is formed at surface and injected as a foam per se. The best process involves injecting the surfactant solution and, once it is in position within the reservoir, then injecting the gas to form the foam.
The use of foams for mobility control has been well documented and described in the patent literature. For example, such a process is described by Worton et. al. in U.S. Pat. No. 2,623,596, utilizing carbon dioxide as a miscible solvent gas. U.S. Pat. No. 3,342,256 by Bernard et. al., describes a process for improving sweep efficiency when injecting water and gas into a reservoir wherein a variety of surfactants have been added to the water. Successful field applications using foams have been reported. Castanier reported the results of 16 field tests of surfactants used in conjunction with steam, both with and without inert gases. (Proc. 4th European Symposium on EOR, Oct. 27-30, Hamburg, West Germany, 1987). Hirasaki reported the results of 10 field tests using steam foams, all of which were successful (Journal of Petroleum Technology, May, 1989, p. 449-456). Smith disclosed the results of 4 successful non-thermal foam floods (ACS Symposium Series 373, Chapter 22, 1987).
The foam exhibits a viscosity which is greater than either the gas or liquid phases of which it is composed. The foam functions by reducing the mobility of the subsequently injected gas in the portions of the reservoir where it is found. The foam accumulates preferentially in the well-swept and/or higher permeability zones of the formation that would otherwise expend a large quantity of the injected gas. Thus, the use of foams will cause the injected gas to be directed to other parts of the formation which had been either unswept or underswept previously.
Recent studies have indicated that the oil phase will influence the stability and performance of foams. It is accepted that crude oils will generally act as a defoaming agent. However, not every foam will be destabilized by the oil phase. Indeed, in some instances exactly the opposite may occur.
Typically, the performance of the foam in a porous medium is determined by its ability to increase the pressure drop across the porous medium.
It is known that fluorocarbon surfactants will function as interfacial tension reducing agents in an oil medium. Additionally, it is known that such surfactants will generate stabilized foams in an oil phase medium. This is exemplified in U.S. Pat. No. 4,836,281, issued to M. Robin and C. Demay. It is further known that amphoteric and anionic hydrocarbon surfactants are extremely oil-sensitive, demonstrating poor stability in the presence of an oil phase.